The present invention relates to improvements in the inspection, assessment and re-working of manufactured components to improve uniformity, reduce the level of physical reworking of the parts and reduce the number of scrap parts. In particular, the invention relates to a virtual alignment system and method for components such as nozzle guide vanes (NGVs) and blades from a gas turbine.
Components such as nozzle guide vanes (NGVs) and blades are often created using precision casting techniques. In particular, investment casting techniques are often used, particularly where complex internal cooling geometry has to be formed within the component. These casting techniques inherently produce levels of dimensional variation that can exceed drawing specifications for the components in question. In order to rectify the dimensional variations from the casting process, as well as to improve the surface finish and bring the parts back into dimensional conformance, it is common for the components to be manually inspected and finished by hand using linishing belts and wheels.
Briefly, the process typically starts with a skilled operator measuring a part for wall section thickness and dimensional conformance with drawings using a known technique such as ultrasonic and/or Coordinate Measuring Machine (CMM). If the part does not conform with the drawings, the operator then has to decide if the component can be reworked either by simply removing all material that lies outside the drawing limits or, if there are areas where material is ‘missing’ in comparison to the drawings, by removing material selectively from the datum locations to bring the overall part back into conformance. If the part is deemed not to be reworkable, it is scrapped.
The input of the operator in the process allows parts that might otherwise be scrapped to be salvaged and reworked. In particular, a part that shows missing, or ‘negative’, material when aligned using the defined datum points for that part would otherwise be scrapped because these voids cannot simply be filled. By using their knowledge of the datum locations together with dimensional data obtained from multiple measuring devices a user can, in many cases, remove material specifically from the datum locations of a part (e.g. a gas turbine NGV or blade) and move the part in 3D space to bring all the features back within drawing tolerances.
The method is reasonably effective in recovering parts that might otherwise be scrapped, but being a manual operation it is highly dependent upon the skill and experience of the individual operator. The operator must use the dimensional data and the knowledge of the part datum system to ensure the external conformance of the component while often also ensuring the integrity of the wall section of the part. This can be extremely difficult, especially for parts with complex core geometries or cooling passages where the wall thickness of the part can vary significantly from one location to another. The user must be extremely careful to maintain a minimum required thickness in all locations without the benefit of any external indication of the local wall thickness during the operation.
Additionally, parts can have complex three dimensional shapes with as many as nine interrelated datum points being used for alignment. Removing material from one location risks moving one or more points outside the drawing tolerances, which would then necessitate an additional reworking operation subject to the same risks as outlined above.
It should be understood, therefore, that the manual operation described above is, by its nature, susceptible to high variation in effectiveness. Even highly skilled operators can struggle due to the large number of interrelated parameters that must be considered and the limited visual feedback during the work. This results in inconsistency of dimensional conformance of the parts within the tolerance bands. This can then lead to decreased performance of the part when used in service.
Furthermore, if insufficient skill or care is used in the re-working process, a theoretically recoverable part will still need to be scrapped because, for example, removal of material from one datum point either moves other parts of the component too far from the drawing tolerances to be recoverable or reduces the wall thickness in an area below the minimum allowed.
Due to the risks associated with removing too much material or removing material from the wrong place, the reworking operation is typically iterative, with several small adjustments being made and the component being re-measured between each adjustment, making the manual inspection and processing inefficient and time consuming.
The manual processing/reworking described above can thus be seen to lead to unnecessary amounts of reworking, which in certain cases can still result in a component that must be scrapped. The linishing belts and wheels typically used for the reworking also expose the operators to hand and arm vibration, so there are additional health and safety issues associated with the manual processing of the components.
It would be beneficial, therefore, if such manual processing could be minimised, or possibly avoided altogether.
It is an aim of the present invention to provide an improved means for inspecting and adapting components for conformity that does not rely so heavily on manual input and expertise.